This invention relates generally to digital computers, and more particularly to instruction decoding in microcode-controlled digital computers. U.S. patent application Ser. No. 441,967, filed Nov. 15, 1982 and assigned to common assignee Data General Corporation is incorporated herein by reference.
Designers of such computers are generally faced with the problem of designing means for decoding or "cracking" each instruction in order to determine the starting address in a control store of a sequence of microinstructions that will effect the operations specified by the instruction. Historically, this has involved a design tradeoff between the size and complexity of the control store on the one hand, and the size and complexity of the cracking logic on the other.
One extreme of the prior art approach to this tradeoff would be to have no cracking logic at all, but to use the instruction directly as the address of the microinstruction sequence; under this approach, a machine using an n-bit instruction would require a control store with at least 2.sup.n locations.
The other extreme of the prior art approaches the "hard-wired" machine which has no control store, and in which control of the operational circuit elements is effected entirely by the instruction-cracking logic circuits; as this extreme is approached the advantages of a microcode-controlled machine, such as ease of altering the machine's characteristics, are lost.
Designers of microcode-controlled machines have worked somewhere between those two extremes, and in recent years have benefitted to some extent from use of integrated circuit technology for constructing cracking logic. Nevertheless, serious problems to operating speed and overall performance still remain. In the prior art trend to reduce the overall size of their machines, attempts to decrease the size and complexity of the control store have been limited by the corresponding increased size of cracking logic built of conventional off-the-shelf integrated circuits.
The present invention offers a solution to these drawbacks and disadvantages of the prior art.